The membrane separation methods include reverse osmosis as well as ultrafiltration, nanofiltration and hyperfiltration, where required also in a combination.
Membrane separation methods are, for example, employed for treatment of water, in particular for desalination of water. Nowadays, the treatment of water becomes increasingly important. Besides a treatment of water to obtain potable water, high-purity process waters are required in particular in the chemical and pharmaceutical industries and need to be prepared on a large scale and as cost-efficiently as possible. Furthermore, high-purity water is also required in the semiconductor industry, for example, to rinse silicon wafers, in particular after etching processes. Generally, the requirements regarding water purity are very high in that field.
The membrane separation method can be combined with other methods of pre-treating the supply liquid and/or post-treating the permeate and/or the concentrate. For example, it is known to further supplement such a procedure by a degassing step prior to or subsequent to the membrane separation method if relatively large amounts of dissolved carbon dioxide are present in a raw water used as supply liquid.
With the membrane separation method and/or the membrane separation device, a feed stream of the liquid to be treated is separated into two partial streams, namely a concentrate and a permeate. The amount of permeate is referred to as permeate capacity or shortly capacity of the membrane separation device. The ratio of the partial streams to one another is referred to as recovery rate or conversion rate. In particular, the ratio between the volumetric flow rate of the permeate (permeate flow rate) and the volumetric flow rate of the concentrate (concentrate flow rate) is referred to as permeate recovery rate.
Due to changes in influencing variables such as age-related changes of permeability of membranes, pressure fluctuations, temperature fluctuations and/or concentration fluctuations of the supply liquid, there may occur changes in the permeate capacity and/or the permeate recovery rate during operation of a membrane separation device without change of the operating parameters.
It could therefore be helpful to provide methods of operating a membrane separation device, control devices for a membrane separation device and devices that operate a membrane separation device in a continuous flow process, wherein such changes in influencing variables can be compensated.